Researchers investigated the link between the use of cell phones and brain cancer. There are two variables in this study: One is the extent of cell phone usage, and the second is whether a person has brain cancer. The extent of cell phone usage was measured and recorded for a group of 35 people who have brain cancer and for a group of 35 people who do not have brain cancer. The type of study executed by these researchers is called a/an: O Experimental Study O Observational Study O Matched-Pairs Study None of these